Solder resists have been conventionally used for manufacturing flexible printed wiring boards and tape carrier packages, including: a type in which a polyimide film referred to as a cover lay film is stamped with a die in accordance with a pattern and then pasted using an adhesive; an ultraviolet ray-curable type in which a film having flexibility is formed; a type in which a thermosetting solder resist ink is applied by screen printing; a type of the a liquid photosolder resist ink for forming a film having flexibility; and the like.
Among those in the foregoing, the cover lay film is incapable of forming a high-precision pattern because of having poor followability to copper foil. On the other hand, the ultraviolet ray-curable solder resist ink and the liquid photosolder resist ink have been still susceptible to improvement in terms of adhesiveness with the polyimide of a substrate and flexibility. Since shrinkage on curing of the solder resist ink and cooling shrinkage after curing are great, such a defect that warpage occurs may also be caused.
In contrast to such an ultraviolet ray-curable solder resist as described above, a solder resist comprising a thermosetting resin composition is generally known to be excellent in adhesiveness with a polyimide substrate and flexibility. Such solder resists comprising thermosetting resin compositions include, e.g., those described in Japanese Patent Publication No. 5-75032 (Claims) and Japanese Patent Laid-Open No. 2006-117922 (Claims).
On the other hand, printed wiring boards and flexible printed wiring boards, which are implemented in various instruments, are used. Therefore, it is demanded that they should also have resistance to sudden environmental changes including those in temperature. Accordingly, a solder resist also requires high resistance to temperature changes; however, when a difference between the coefficients of liner thermal expansion (CTE) of a thermosetting resin and a substrate or a substrate-forming material such as copper or an underfill is great, there is a problem that cracks are generated in the resist in TCT (thermal cycle test).
In contrast, in recent years, the CTE of a thermosetting resin has been widely matched with the CTE of a neighboring member material. For example, CTE can be decreased by highly filling a thermosetting resin with an inorganic filler.